#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <climits>
using namespace std;

typedef unsigned long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

#define INF INT_MAX/3
#define REP(i,n) for(int i=0;i<n;i++)

ll N,K;


ll Zyun(ll n,ll r){
	return r!=0 ? n*Zyun(n-1,r-1) : 1;
}
ll fact(ll n){
	return n!=0 ? n*fact(n-1) : 1;
}


int main(){
	ll sum = 1;
	cin>>N>>K;
	
	if(K==1){
		cout<<"0"<<endl;
		return 0;
	}
	
	ll p = N-K;	//p = 2
	for(ll i = 1;i<=p;i++){
		int c;
		if(i > p/2){
			c = p - i;
		}else{
			c = i;
		}
		sum += Zyun(p,c) / fact(c);
	
		// fact(2,0)/fact(0,0)
	}
	cout<<sum<<endl;
		return 0;
}